My fist time through the game I found a pair of leather boots with the muffle enchant on them and that is how I got it. You cannot disenchant an piece of special equipment with the enchant on it how ever. Its not a bug, for example the boots you get for joining the Dark Brotherhood have the enchant with muffle on it but you cant disenchant those. Its all prety much random and requires a rather large amount of luck.

I'm with Steve on this one, I got a pair early on in the game. I'm actually on my 4th character (all over level 40) and have found a pair of steel boots with muffle on them that could be disenchanted 1 time...out of four playthroughs! Bought em from Adriana in Whitrun. They're incredibly rare.

The magnitude of the Muffle effect is significant. However, the magnitude of enchantments and spells is not displayed in-game. Most built-in enchanted items have a magnitude of 0.5, meaning they are only 50% effective.
The actual effectiveness of the Muffle effect for magnitudes below 1.0 depends on armor weight, i.e. 50% of the noise of Daedric is still much higher than 100% of the noise of Elven.

All custom-enchanted Muffle effects have a magnitude of 1.0, regardless of skill level or soul gem used, making them greatly superior to most built-in enchanted Muffle items.
Muffle also seems to affect the sounds of your weapons (excluding bows) and not just your footsteps.

The best way, still, trying to drop, find in shop or via console a pair of Steel boots of Muffling

After about 200 hours of playing, with 3 different characters I bought ebony boots with the muffle enchantment on them in whiterun. It's the only time I've ever seen boots with this enchantment on them that you could disenchant, so good luck ;-)

Muffled boots are easier to get around level 11. The higher you get in level the less chance they have of appearing. Planning to get it ahead of using it is most effective.

Muffle is superior to sneak enchantments because even with 100 sneak, vampirism (25% sneak bonus), muffled movement and invisibility will still allow you to be "sensed" due to the noise heavy armor makes. With muffled movement and muffle though light is the only weakness to stealth. Though muffled movement works well with the vampire bonus unless you try to kill an enemy standing in the same room as another one.

Without muffle many enemies will draw their weapons and move around as if searching for you even though the eye is fully closed. Muffled movement, muffle (custom enchant, not unique), muffle (spell) and invisibility will get you through any area undetected though.

Another advantage of invisibilty over normal stealth mechanics is you can steal in bright light by grabbing the item (hold A on the xbox) and drag it into shadows before taking it and then recast invisibility to enter the light again. Grabbing does not break invisibility so even bodies can be looted safely this way. Stealth and illusion magic with quiet casting is the best form of stealth because invisibility removes lighting from the equation entirely. A little bit of light is all it takes to be seen from significant distances regardless of your skill and enchantments.

As an example line the steps to Vlindrel Hall in Markarth with a few torches after reaching 100 skill, plus enchantments and then try to sneak from the bottom to the top unseen by the guard at the guard tower behind the temple of Dibella. It is impossible without invisibility (even with shadow warrior). Finally preparing invisibility prior to walking through a transition and then releasing it during the loading screen will immediately cast it before anyone on the other side can usually fully detect you (even in daylight).